1. Field of the Invention
This invention relates to the field of reservoir characterization. More particularly, this invention relates to an improved technique for transmitting and/or receiving signals through a tubular such as casing or a liner to measure reservoir characteristics and to place wells with greater accuracy.
2. Description of Related Art
Petroleum is usually produced from oil reservoirs sufficiently far below a gas cap and above an aquifer. As the oil zone is being produced and depleted, the gas cap starts coning downward and the aquifer coning upwards towards the oil bearing zone. Such migration can adversely affect the extraction of petroleum by creating pockets that are missed by the producer and by saturating the oil deposits with water. As soon as either gas or water hits the well, its oil production usually ceases instantly.
Reservoirs are monitored for changes in saturation and early signatures of coning so that corrective action can be taken. One approach utilizes pulsed neutron measurements, which measure formation sigma (indicative of saline water) or carbon-oxygen ratios (indicative of the ratio of hydrocarbon to water). The primary disadvantages of such pulsed neutron measurements are shallow depth of investigation and low accuracy in low porosities. A shallow measurement can be fooled by water channeling behind casing, and shallow re-invasion of well fluids into the open zones (e.g., the perforated zones) when the well is not flowing.
Measuring the electrical resistivity near a borehole has long been used to determine production zones in oil and gas fields and to map sand and shale layers. Electrical resistivity depends directly on porosity, pore-fluid resistivity, and saturation. Porous formations having high resistivity generally indicate the presence of hydrocarbons, while low-resistivity formations are generally water saturated.
Cross-well monitoring is an approach to monitoring changes in reservoir saturation. This technique is an outgrowth of radar experiments conducted in the early 1970s. See Michael Wilt, Exploring Oil Fields with Crosshole Electromagnetic Induction, SCIENCE AND TECHNOLOGY REVIEW, August 1996 (available at <http://www.llnl.gov/str/Wilt.htm>); See also Q. Zhou et al., Reservoir Monitoring with Interwell Electromagnetic Imaging, SPWLA FORTIETH ANNUAL LOGGING SYMPOSIUM, May 30-Jun. 3, 1999. With this technique, a transmitter is deployed in one well and a receiver is deployed in a second well. At the receiver borehole, the receiver detects components of the transmitted and induced currents for determination of the reservoir characteristics between the wells.
This approach has been studied for wells with fiberglass casing. The technique was used to monitor water-saturation changes in heavy oil zones undergoing steam flooding. See Michael Wilt et al., Crosshole electromagnetic topography: A new technology for oil field characterization, THE LEADING EDGE, March 1995, at 173-77. However, this technique is presently limited to closely spaced wells with either open-hole completions or cased with insulating composites. The disadvantages of these systems include the fragility and expense of fiberglass casing, making the technique impractical for use in production wells. Moreover, drilling a special well for monitoring is very expensive and therefore rarely done.
Another proposal for surveying and monitoring a reservoir is to deploy electrodes on the exterior of the casing. U.S. Pat. No. 5,642,051 (assigned to the present assignee) describes a casing, which has external insulation, electrodes, and cables for use in the completion. Its disadvantages include: the fragility of the external hardware and cable, the difficulty of running a complex completion into the well, the logistics of running a wire outside of the casing from surface to downhole, the inability to repair damaged or malfunctioning components, the difficulty to guarantee a hydraulic seal between the casing and the formation with external cables present, the possibility of cross-talk between these cables, the difficulty to place preamplifiers and other electronics near the electrodes, electrode impedance effects, and the influence of the cement annulus on resistivity.
Downhole techniques have been proposed utilizing slotted tubes or slotted liners. U.S. Pat. No. 5,372,208 describes the use of slotted tube sections as part of a drill string to sample ground water during drilling. A Slotted liner is a completion method used to prevent the wellbore from collapsing, and may also be used to prevent sand grains and other small particles from entering the wellbore and forming debris piles which may restrict fluid flow. A slotted liner is most often used in a horizontal well and is within a single producing formation. It is an alternative to leaving the hole completely open (i.e., with no casing), when an open hole may collapse or become blocked with debris. However, these types of slotted tubes or liners are not cemented in the wellbore, and do not provide hydraulic isolation from one well section to another. Slotted liners may be obtained from manufacturers such as Valley Perforating Co. of Bakersfield, Calif. (information available at <http://www.valleyperf.com/perf.htm>). See also James J. Smolen, Production Logging In Horizontal Wells, SPWLA THIRTY-FIFTH ANNUAL SYMPOSIUM, workshop notes, Tulsa, Okla., Jun. 19, 1994.
These technologies have not been readily applicable to measurement and monitoring techniques using steel-cased production wells. The steel casing dramatically attenuates electromagnetic (“EM”) signals, restricting the possible use of known techniques primarily to qualitative detection of high resistive zones, but not for quantitative saturation measurements. For the cross-well technique to be successful in cased holes, it has been proposed to run at extremely low frequencies so that the magnetic fields can penetrate steel casing. However, such an approach is extremely sensitive to the magnetic and electric properties of the casing, and it has not been successfully demonstrated.
Thus, there remains a need for a technique for measuring and/or monitoring the characteristics of subterranean formations through steel casing. It is desirable to implement a technique that provides transparency for the passage of a signal through a tubular such as steel casing while maintaining hydraulic isolation between the tubular and the surrounding formations. It is also desired to implement a technique for through-casing signal-transmission and/or reception to place wells with greater accuracy.